Various methods for recognizing misfires in an internal combustion engine are already known. These include methods which receive the pressure or light signals within various cylinders and supply them to a common control unit for evaluation. Since the corresponding arrangements are complex, they have not, up to now, been installed in series manufactured vehicles.
Particularly in vehicles equipped with a catalyzer, however, it is desirable to continuously monitor for misfires. This is because misfires cause unburnt fuel to reach the catalyzer where it can ignite. This often leads to the catalyzer being burnt out and, in particularly severe cases, to a vehicle fire.
U.S. Pat. No. 3,965,677 discloses a method which detects such faults by monitoring the exhaust-gas pressure signal and, in the case of deviations from an expected variation, takes measures to protect the catalyzer. In this method, the maximum and minimum of the exhaust-gas pressure amplitude are measured within a specified time duration, that is, over a plurality of periods of the exhaust-gas pressure signal. If, after a correction to compensate for intake pipe under pressure effects, the difference of these values lies above a threshold, this is interpreted as an indication of a malfunction and the exhaust gases are then diverted around the catalyzer.
U.S. Pat. No. 3,977,239 discloses a further method which utilizes the periodic fluctuation of the exhaust-gas pressure for purposes of diagnosis. The pressure variation in the exhaust-gas line as a function of time is converted into an electrical signal and subjected to spectral analysis. In addition to a frequency component which is attributable to the ignition frequency, other components can occur which are characteristic of misfires. A comparison of the amplitudes provides a measure of the unevenness of the power strokes.
A similar procedure is disclosed in U.S. Pat. No. 4,189,940. Here, the pressure in the crankcase is measured as a further variable in addition to the pressure signal of the exhaust gas. Both pressure signals are split spectrally into frequency components after conversion into electrical quantities. The amplitudes of these components are then compared for diagnostic purposes.
Common to the three last-mentioned methods is the fact that the pressures are not measured for a single exhaust-gas pressure fluctuation. This represents a defect, for example, if not only the occurrence of a fault but also its localization is desired. For example, the methods mentioned cannot associate a fault in a particular cylinder with this cylinder since the pressures of the exhaust-gas pressure fluctuation associated with a particular ignition are not detected individually.